Pre-activation treatment in the electroless plating of synthetic resin substrates

ABSTRACT

AQUEOUS DILUTE SURFACTANT SOLUTIONS ARE EMPLOYED IN THE TREATMENT OF PLASTIC SUBSTRATES TO PREPARE THEM FOR ELECTROLESS PLAING TO ENSURE COMPLETE COVERAGE OF THE SUBSTRATE SURFACE BY THE METAL. THE SUBSTRATES ARE IMMERSED IN THE SUFACTANT SOLUTION AS A PRELIMINARY STEP IMMEDIATELY PRECEDING THE ACTIVATION OF THE SUBSTRATE IN THE NORMAL CYCLE OF ELECTROLESS PLATING OPERATIONS, SUPPLANTING VARIOUS OTHER PRE-ACTIVATION STEPS AND, UNLIKE SUCH PRIOR STEPS, BEING OPERATIVE FOR ALL TYPES OF SYNTHETIC RESINS ENCOUNTERED IN ELECTROLESS PLATING OPERATIONS.

United States Patent 0 3,563,784 PRE-ACTIVATION TREATMENT IN THE ELEC- TROLESS PLATING OF SYNTHETIC RESlN SUBSTRATES 1 William P. Innes, Cheshire, Eugene D. DOttavro, Thomaston, and Sharon D. Brown, Waterbury, Conn., assignors to MacDermid Incorporated, Waterbury, Conn., a corporation of Connecticut No Drawing. Filed Sept. 9, 1968, Ser. No. 758,589 Int. Cl. C23c 3/00; B44d 1/40 US. Cl. 117-47 4 Claims ABSTRACT OF THE DISCLOSURE Aqueous dilute surfactant solutions are employed in the treatment of plastic substrates to prepare them for electroless plating to ensure complete coverage of the substrate surface by the metal. The substrates are immersed in the surfactant solution as a preliminary step immediately preceding the activation of the substrate in the normal cycle of electroless plating operations, supplanting various other pre-activation steps and, unlike such prior steps, being operative for all types of synthetic resins encountered in electroless plating operations.

This invention relates to the electroless deposition of a plate or film of metal by chemical reduction of the selected metal from a solution of one of its soluble salts. In particular, the invention here is concerned with a novel preliminary step in the preparation of the surface of the substrate in the sequence of operations involved in a commercially practical electroless plating operation.

In electroless plating of non-conductive substrates, difficulty is often experienced in commercial plating operations because of skips or incomplete coverage of the surface of the substrate by the metal plate, especially at the gate area of injection molded parts or other stressed areas of the plastic substrate. Much work has been done in trying to overcome this difliculty. Preparation of the surface of the substrate prior to the step of actually depositing the metal film thereon commonly includes, first, etching the substrate surface with a chromic-sulfuric acid solution to produce a condition more receptive to the activating or catalyzing operation. This step commonly directly precedes the electroless deposition step whereby minute foci of a catalytic metal are caused to adhere to the surface and serve as points of initiation for deposit of the plating metal during the plating operation. Refinements in the procedure have included subjecting the substrate to a supplemental operation between etching and activation of its surface. Such supplemental operation has involved removal and/or reduction of adherent or trapped residual hexavalent chromium on the substrate surface before activation thereof, using reducing agents such as stannous chloride. This step is known in the industry as sensitization of the plastic surface. Reducers other than stannous chloride have also been tried, for example sodium sulfite. Another procedure has been to supplement the usual chromic-sulfuric acid etch treatment with a phosphoric acid dip prior to activation. Each of these procedures produces some improvement in plating of certain plastics but no one of them is generally applicable to the wide variety of different synthetic resin materials commonly employed in commercial plastic molding operations today. It is important for commercial plating installations to be able to operate the complete process from initial etch to final plate on a continuous, automatic or at least semi-automatic cycle. Desirably parts to be plated are suspended on conveyor-borne racks and successively progressed Without re-racking through treatment tanks containing the different treatment solutions necessary to effect the desired chemical deposit of metal on the substrate. In some cases it may even be desirable to extend the automatic cycle all the way through the state of electrolytic plating which generally follows chemical plating in order to build up a metal film of desired final thickness. Where different plastic substrates are plated in the course of an operating day or on different days in a commercial plating installation, the requirement for different treatment steps and compositions for one type of plastic from that of another presents scheduling problems as well as less than maximum use of the different treatment solutions required.

It is accordingly a primary object of the present invention to simplify and standardize the pretreatment procedure leading up to the electroless deposition of a metal on a plastic substrate by providing a sequence of steps or operations and treatment solutions which remain the same and are fully effective for any of the different types of synthetic molded resins normally processed.

In brief, this objective is achieved by means of a preactivation step employing a surfactant in prescribed concentration in aqueous solution and for a given time of immersion of the substrate in the solution. This preactivation step is interposed between the normal etching operation and the surface activating or catalyzing operation in the conventional plating cycle described above. The preactivation bath used is effective in getting complete surface activation and consequently complete surface coverage by the electroless plating metal for any of the commonly used synthetic resins encountered today in commercial molding operations.

The exact mechanism of the surfactant solution treatment at this stage in the process is not fully known or understood. However it serves at least the following purposes. First it acts as any surfactant will to decrease surface tension on the substrate and improve wetability. Surface tension depression alone is not enough however, as many surfactants having good tension depression activity do not produce the uniform, continuous deposits of metal which characterizes the use of the particular surfactants hereinafter disclosed. Finally, the selected preactivation surfactant acts as a chrome kill. Other known hexavalent chromium reducers such as sodium sulfite,

stannous chloride and hydrosulfite reducers do not act in an equivalent manner with all plastics. Thus the multipurpose function of the designated surfactants appears to be a unique property.

The surfactant materials found to be operable for the purposes of the present invention are designated in the following table which also lists other pertinent information on their composition and the approximate overall and preferred operating ranges of concentration in aqueous solution.

ably in separate rinsing tanks and immersed in a preactivation solution consisting of plain (deionized) water TABLE I Concentration (p.p.1n.)

Surfactant Manufacturer Chemical characteristics Operating Preferred "Aliquat-QG. .1 General Mills A tallow trimethyl ammonium chloride 3. 5-1, 000 -15 Igepal Co 030 General Aniline Film Corp... Nonylphenoxypoly (ethyleneoxy) ethanol-.. 4-1, 000 8-12 Alkapent M-10N. Wayland Chem Acid phosphate esters of the general group- 10-1, 000 80-120 R =alkylphenol or aliphatic group.

A typical electroless plating cycle incorporating the novel pre-activation step comprises the following sequence of operations.

EXAMPLE 1 An automotive seat belt buckle of molded polypropylene is immersed in a preplating aqueous emulsion of turpentine and emulsifying surfactant at 150 F. for minutes. The turpentine emulsion is more fully disclosed in a copending application of Grunwald and DOttavio, Ser. No. 654,901, filed June 14, 1967. In general, the emulsion comprises an aqueous solution of turpentine and surfactant materials constituting a micro or macroemulsion. Particularly satisfactory surfactants are a mixture of IGEPAL C0630 and BENAX 2A1. The former which happens to be the same surfactant used again later in the novel preactivation step of this invention is the trade name of General Aniline and Film Corporation for alkylphenoxypoly (ethyleneoxy) ethanols produced by reacting alkylphenol with ethyleneoxide and having the general formula RC H O(CH CH O) CH CH OH, where R is Cal I17 or a higher homolog. BENAX 2A1 is the trade name of Dow Chemical Co. for sodium dodecyl diphenyl ether disulfonate.

The buckle is next carefully rinsed in water and then immersed in a chrome-sulfuric acid etchant solution containing approximately 14% chromic acid, 40% sulfuric acid, (44 Be), at 175 F. for 5 minutes. Again the buckle is carefully rinsed, after which it is then immersed in the novel preactivation solution of this invention. Good results are obtained when such preactivation solution consists simply of plain (deionized) water containing about 8 parts per million of IGEPAL C0630 at a solution temperature of 80 F. for an immersion time of about 2 minutes. The buckle is then carefully rinsed and activated for nickel deposition, for example, by any of the known activating procedures, including the two-step stannous chloride-palladium chloride treatment or the one-step acid tin-palladium hydrosol treatment. Again the buckle is carefully rinsed and preferably subjected to an accelerator solution comprising a 10% aqueous solution of fluoroboric acid at 75 F. for 1 minute. The buckle is carefully rinsed and then immersed in an electroless nickel plating solution of conventional composition. A satisfactory nickel plating solution is described in US. Pat. No. 2,532,283, Example V, Table I; in addition there are many proprietary electroless plating compositions commercially available and these are quite satisfactory in practicing the present invention. Thereafter the part is thoroughly rinsed and subjected to electrolytic plating in a nickel bath to build up the nickel deposit on the article. A good adherent plate of nickel, free of skips, is produced.

EXAMPLE 2 A molded polystyrene resin plaque is prepared for copper plating, for example, by immersing it in an acid cleaner for about 1 minute and then etching it in the chromic-sulfuric acid etchant solution as described in Example 1. The plaque is then carefully rinsed, prefercontaining about 7 ppm. of ALIQUAT-26. It is then activated and accelerated, as in the previous example, followed by electroless plating using in this case any of the electroless copper plating solutions commercially available. A typical example of a satisfactory copper solution is described in US. Pat. No. 3,095,309, Example I. Again electroplating of additional copper is usual to build up the thickness on the plate. The resulting copper plate is skip-free and has acceptable bond strength.

EXAMPLE 3 A phenolic molding was prepared for nickel plating by following the same acid clean and sulfuric-chromic acid etchant treatments described in Example 2. As a preactivation treatment, the molding was immersed for 2 minutes in an aqueous solution consisting of plan (deionized) water containing p.p.m. of ALKAPENT M-lON. This step was again followed by activation and acceleration, and then by electroless nickel plating and reinforcement by electroplating. The plate is skip-free.

It has been found that essentially uniform skip-free plating is achieved with any of the preactivation solutions listed in Table I for any of the commonly plated resin substrates using the same cycle of preparatory steps outlined in the examples above. This includes, in addition to the polystyrene, polypropylene and phenolic substrates mentioned in the examples, other polymers such as polyethylene and polybutylene, as well as urea formaldehyde and epoxy resins.

Concentration of the surfactant material in the preactivation solution is critical in the sense that not only is is necessary to have sufiicient of the surfactant material present to be effective, but there is at the same time an upper limit above which rack plating occurs, i.e. metal begins to deposit on the conveyor racks themselves as well as the articles to be plated. The spread between the lower and upper limits of concentration is quite narrow, being on the order of a minimum of around 5 ppm. to a maximum of 1000 ppm. As indicated in Table I, the preferred operating range for all of the useful surfactant materials except ALKAPENT M-lON is nearer the lower rather than the higher limit.

The pH of the preactivation solutions can be varied from 2 to 11 with no apparent effect on the performance of the bath, so long as the surfactant remains soluble in the solution.

Temperature of the preactivation bath may vary so long as this does not exceed the boiling point of the solution or the cloud point of the surfactant. Within this range, activity of the preactivating solutions increases with temperature with the result that shorter immersion times can be used.

What is claimed is:

1. In a pocess of plating a synthetic resin substrate with copper or nickel by electroless deposition from an aqueous solution containing a soluble salt of the selected metal, the combination of steps which comprises:

(a) subjecting the surface of the resin to a preplating conditioner comprising a finely dispersed clear stable aqueous emulsion of turpentine and a surfactant,

(b) rinsing the surface of said substrate in water and then subjecting it to contact with an aqueous oxidizing solution of chromic and sulfuric acids;

() rinsing the surface of the substrate again in water and subjecting it to contact with a dilute aqueous solution of a surfactant selected from the group consisting of tallow trimethyl ammonium chloride, nonylphenoxypoly (ethyleneoxy) ethanol, and acid phosphate esters having the general formula wherein R is an alkylphenol or an aliphatic group, and wherein the concentration of said surfactant in soltuion is at least ppm. but not in excess of 1000 p.p.rn.;

(d) rinsing the substrate again in water and activating the surface thereof by subjecting it to contact with a hydrochloric acidstannous chloride hydrosol of palladium colloidally dispersed therein; (e) rinsing the substrate and subjecting it to contact with a dilute aqueous solution of fluoroboric acid; (f) rinsing again and subjecting the substrate to contact with an electroless plating solution containing a soluble salt of the metal to be plated and a reducing agent therefor to effect chemical deposition of the metal onto the substrate.

2. The process as defined in claim 1, wherein the surfactant is tallow trimethyl ammonium chloride at a concentration of about 7 ppm. and contact with the substrate is maintained for 2 minutes.

3. The process as defined in claim 1, wherein the surfactant is nonylphenoxy(ethyleneoxy)ethanol at a concentration of about 8 ppm. and contact with the substrate is maintained for about 2 minutes.

4. The process as defined in claim 1, wherein said substrate is maintained in contact with said preactivating solution for a period of at least 5 seconds at ambient room temperature.

References Cited UNITED STATES PATENTS 2,999,767 9/1961 Clay l06lX 3,033,703 5/1962 Schneble 117-47 3,246,995 4/1966 Moore l061 3,300,335 1/1967 Horvath 1061X 3,326,700 6/1967 Zeblisky l061 3,347,724 10/1967 Schneble l06lX 3,438,798 4/1969 Baudrand l17-47 WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R.

P0405" UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent n 3,563,784 Dated February 16 1971 Inventor) William P. Innes, Eugene D. DOttavio and Sharon D.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

' Column 3, line 27, application Ser. No. 654,901 should be changed to read 645,901

Signed and sealed this 114th day of September 1971 (SEAL) Attest:

EDWARD M-FLETCHER,JR. ROBERT GOTTSCHALK Attestlng Officer Acting Commissioner of Pater 

